The injection of auxiliary fuels (natural gas, oil, coal or other carbonaceous materials) into the blast furnace has been driven by economic factors. Mid of last century, oil was, due to its low price, the preferred auxiliary fuel to reduce the consumption of expensive metallurgical coking coals and to avoid capital expenditures linked to the expansion of the coke-making plant.
A first major re-evaluation considering auxiliary fuel injection had to be done due to the oil crisis in the 1970s. Although pulverized coal injection had been practiced in some blast furnaces since the early 1960s, it was only in the 1980s that the interest for PCI escalated due to the oil price shocks.
A second, more recent re-evaluation of auxiliary fuel injected into the blast furnace is caused by the drastically increasing energy prices, including the natural gas price and the decoupled price evolution for non-coking coals. Due to the higher availability, it is much probable that the non-coking coal prices will, also in future, remain lower than those for oil and natural gas.
It is well known that the injection of fuel, such as e.g. pulverized or granular coal, into the hot-air blast, which is blown through a plurality of tuyeres into a lower portion of the blast furnace, has many advantages. In particular, the injection of coal decreases the overall cost of produced hot metal, not only through the replacement of coke, but also through an increased productivity and the possibility of a prompt control of the blast furnace operation.
The injection of pulverized or granular coal is performed conventionally by means of a fuel injection lance into the hot-air blast at a certain distance upstream from the tuyere end opening into the furnace. In other words, the coal is injected through the hot-air passage in the tuyere. The coal fed through the fuel injection lance is in suspension in a transport gas.
Regarding all the economical and ecological advantages of coal injection, the injection levels will continue to rise. A major concern related to higher injection levels is the combustion behavior of the coal in the blast furnace. Inefficient coal combustion in the raceway will result in unburned coal particles obstructing the permeability within the void spaces of the burden and thus causing a degraded blast furnace operation leading to production losses.
In order to minimize the char load into the blast furnace the coal combustion within the raceway has to be maximized. This can be done by an improved mixing of the well dispersed pulverized coal with the oxygen enriched hot gas. As the residence time of the coal particles in the raceway is only in the range of a few milliseconds, it is important to reach the ignition point very rapidly.
The ignition point of a specific coal is dependent of the coal type and its size distribution, and of parameters like for instance the oxygen enrichment as well as the hot blast, the oxygen, the coal transport gas and the coal temperature.
As more fuel is fed into the blast furnace, the quantity of oxidizing gas has to be increased in order to warrant a correct burning of the additional fuel. Typically, the additional oxidizing gas is fed through a separate gas injection lance having its gas outlet in the vicinity of the outlet of the fuel injection lance. Alternatively, the combined injection of fuel and oxidizing gas has been suggested e.g. in EP 0 447 908, wherein the injection is performed through a coaxial lance, wherein an outer tube is arranged surrounding an inner tube. The inner tube forms a separation wall between the oxidizing gas and the fuel until both reach an outlet nozzle of the lance. Such coaxial injection lances are often referred to as oxycoal lances. In EP 0 447 908, oxidizing gas is conveyed in the outer tube and fuel is conveyed in the inner tube.
A disadvantage of these systems is that the oxidizing gas fed through the separate gas injection lance or the oxycoal lance is cold. Consequently, when the oxidizing gas meets the fuel, ignition and combustion of the fuel does not take place until an ignition temperature of a mixture of oxidizing gas and fuel has been reached.
It has also been suggested to increase the oxygen content in the hot blast air by increasing the oxygen content in the cold blast air before the latter is heated up in a hot stove. By feeding additional oxidizing gas through the hot stove, the oxidizing gas is heated and can be delivered via the blowpipe to the fuel at a higher temperature. However, high oxygen concentration in the hot blast air may lead to seals and other metallic parts being burnt. The risk of fire increases with higher oxygen concentrations. Typically, the oxygen flow rates in the hot blast air are therefore limited to about 30%. In order to improve the combustion conditions of the fuel, higher oxygen concentrations may however be desired.